It is well-known that the incorporation, in various manner, of a burnable absorber with nuclear fuel pellets, which enables the use of excessive amounts of fuel in a reactor during the initial life of the fuel, can extend the life of the fuel elements. In some instances, the burnable absorber is mixed directly with the fuel and integrated therewith, while in other instances, a burnable absorber coating may be applied to the surface of fuel pellets, or discrete forms of a burnable absorber may be interspersed between conventional fuel pellets, or otherwise located within the cladding for the nuclear fuel.
In U.S. Pat. No. 3,122,509, for example, coherent nuclear fuel elements are disclosed where conventional base glass compositions are admixed with a nuclear fuel or other nuclear material, as a binder glass. Boron carbide, for example, may be blended with a base glass of conventional glass-making materials, and the blended mixture placed in a metal cladding tube and compacted. The compact is then heated to sinter the glass and boron carbide together in a coherent form. The coherent conventional glass-boron carbide cylinders formed were heated to give cylinders that showed a uniform distribution of boron carbide within the cylinders without voids. In effect, this reference discloses the use of conventional glass components for use as a binder, in the formation of an actinide oxide, or other additives, in the formation of fuel elements, or other elements, such as cylindrical fuel shapes, or formation of cylindrical burnable absorber shapes.
Since problems exist with the formation and use of fuel elements that incorporate a burnable absorber directly within the pellet or element, it has been proposed to provide the burnable absorber in the form of a coating on the pellet. As disclosed in U.S. Pat. No. 3,427,222, which is assigned to the assignee of the present invention, a sintered nuclear fuel pellet may have a coating of boron carbide, or other burnable absorber, formed thereon by fusion bonding of the burnable absorber, as a coating, on the surface of the fuel pellet. As disclosed therein, the coating may be formed by (1) plasma spraying or flame spraying of the burnable absorber with optional addition of uranium dioxide or other ceramic oxides, (2) dipping the pellets in a slurry of the burnable absorber and a ceramic binder such as zirconium silicate or sodium tetraborate and firing, (3) vapor coating the pellets, or (4) electron beam bombardment of a burnable absorber on the pellets. Coatings formed by application of a mixture of boron carbide with a binder such as zirconium silicate or by application of sodium tetraborate, however, have resultant problems. Where a coating of a mixture of boron carbide and zirconium silicate was formed, the resultant coating was not adequately stable, and peel tests, using Scotch tape, resulted in removal of the coating. If higher temperatures, in excess of 1000.degree. C. were to be used to sinter the material to improve adhesion, the boron carbide will react with both the uranium dioxide of the pellets and possibly with the zirconium silicate resulting in loss of boron as an absorber in the finished pellet. Where a coating of sodium tetraborate was formed, while the resultant coating showed excellent bonding and passed a peel test, the coatings were not moisture resistant and hydrogen pickup was evident indicating significant moisture adsorption. In addition, the coating has a low melting point (.ltoreq.750.degree. C.) and during normal reactor operation, it would be expected to slump and possible adhesion/reaction of the coating with the cladding will occur.
The present inventors are also aware of the earlier work disclosed in applications, "Burnable Absorber Coated Nuclear Fuel", Ser. No. 468,788, filed Feb. 22, 1983 in the names of K. C. Radford and B. H. Parks; and "Coating a Uranium Dioxide Nuclear Fuel with a Zirconium Diboride Burnable Poison", Ser. No. 468,743, also filed Feb. 22, 1983 in the name of Walston Chubb, which relate to coated nuclear fuel pellets, both of which applications are assigned to the assignee of the present invention.
It is an object of the present invention to provide a glaze coating composition for use in coating of nuclear fuel pellets with a boron-containing burnable absorber which forms a coating having a high melting point and a low propensity for moisture adsorption.
It is another object of the present invention to provide a method for forming nuclear fuel pellets having a burnable absorber combined therewith in the form of a coating of a boron-containing burnable absorber encapsulated in a specially developed boron-containing glass composition.